A variety of plastic films are used in the packaging industry for storage and shipping of goods. In many cases these plastic films are printed with text and images. Printing inks used for this purpose must meet certain end-use requirements for successful use in this application. These requirements include good printability and resistance properties. Inks used with plastic packaging must also exhibit good wetting and flow properties and fast drying times.
Due to environmental and health concerns about solvent based inks, increased emphasis has been placed on the use of water-based inks for printing on films used for packaging in recent years. Water-based inks with good printability (i.e., good ink transfer and wetting, good adhesion to substrate and good image resolution) are highly desirable. Similarly, water-based inks which have good resolubility (i.e. the ability of dry ink to re-disperse in the same ink when in a wet state) are also important. For example, if a press is stopped, as water-based inks begin to dry they can undergo physical and chemical changes due to flocculation, film-coalescence, film formation, change in pH, and the like, which render these inks difficult to resolubilize. Typically, inks with good printability characteristics also have good resolubilizing properties, while inks with good resolubility do not necessarily have good printability properties. Finally, resistance of the printed or coated film to mechanical forces, water, solvents, and other chemicals is also highly desirable. However, many previous water-borne polymer coatings suffer from problems with adhesion, abrasion resistance, and water and solvent resistance.
Various multivalent metal crosslinking agents have been used as additives in water-based coatings in an effort to improve the resistance properties of the coatings. In particular, zirconium based crosslinkers have been promoted as useful additives to water-borne polymer compositions, offering high solvent and abrasion resistance. The basis of the improved wet resistance properties is believed to be linkages formed by the metal species between the polymer molecules at carboxylate and hydroxyl groups. More specifically, it is believed that the carboxylate groups are directly bound to, e.g., the zirconium cations upon evaporation of water and other volatile constituents. The reaction is irreversible under normal coating conditions. In contrast, hydroxyl groups on the polymer likely hydrogen bond with hydroxyl groups attached to the zirconium ion. In either case, the exact nature of the metal crosslinking species (usually in the form of polymeric complexes) will depend on the amount of metal crosslinker present, the nature of the polymer, and the pH of the system, among other variables.
Despite their advantages, metal crosslinkers such as zirconium crosslinkers suffer from chemical and colloidal stability problems. For example, ammonium zirconium carbonate (AZC) is susceptible to oxolation, a process through which an insoluble polymer of zirconium oxide is formed. This problem is exacerbated at dilute concentrations of AZC such as typically would be used with a water-borne polymer. In fact, a manufacturer of an aqueous solution of AZC (BACOTE 20, having 14.8% Zr metal) recommends that AZC be added as the last ingredient to a formulation and not be mixed with any other material or otherwise diluted prior to use. The instability of zirconium agents such as AZC severely limits their use in water-based coatings.
Thus there is a need for water-based coatings, especially inks, which have good wetting, drying speed, flow behavior, printability, and resistance properties when used for printing on plastic films and other substrates. Furthermore, such coatings must be stable upon storage and under normal conditions of use. The water-based inks and coatings of this invention exhibit these and other desirable properties when used on plastic films and other substrates.